The chassis frame in trucks is constructed in a manner permitting it to turn in order to partly participate in the wheel axle movements into different angular positions and inclinations. The torsional angle of the chassis frame relative to the front and rear wheel undercarriages during vehicle operation is dependent on the nature of the road surface and may be quite considerable. When a loading crane is mounted on the truck it is necessary that the chassis frame is given a possibility to turn below the crane base which is rigid.
To meet this requirement the frame attachment on one side of the crane base is in the form of a beam extending transversely across the chassis frame in the form of a so called support bridge in which the crane base is mounted by means of a rotational hub. In addition, the crane base is rigidly attached at two points to the chassis frame side members. According to this arrangement the rotational centre of the chassis frame will, however, be positioned a considerable distance below the mounting point of the support bridge on the crane foot, which, upon turning of the chassis frame, gives a lever effect with the result that the crane base attachments in the chassis frame will be exposed to very large shearing strains.
A theoretical evaluation of the lever movement which is caused by the maximal turning of the chassis frame within the longitudinal section between the attachment points of the crane base reveals a relative movement between the support bridge and the crane base of 5 millimeters or more in each direction. The result is wear on the bridge hub. The attachment screws of the chassis frame come loose, the crane base is exposed to heavy torsional stress, and the chassis frame may be damaged.